1. Field of the Invention
This invention relates generally to a battery charging system for rapidly charging batteries having different voltage ratings and, in particular, to a battery charging system incorporating a logarithmic analog-to-digital converter which provides automatic scaling of the battery voltage signal.
2. Background of the Art
Disposable batteries have a common and widespread use in our everyday lives. From battery-operated toys to battery-operated tools, batteries play an important role. However, batteries are not always reliable. Problems range from batteries which lose their power from inactivity to batteries which seem to lose their power at inopportune times from continued use. Therefore, it is generally desirable to keep fresh batteries on hand. With the number of different styles and voltage outputs of so many different batteries it is typically quite a task to keep the right batteries stored. In addition, batteries can be relatively expensive to purchase, and therefore are not always conveniently at hand. Moreover, there are a number of environmental concerns about disposing of batteries in landfills and the like due to the toxicity of many battery materials.
Because of at least these characteristics of disposable batteries, the popularity of rechargeable batteries is on the rise. For a rechargeable battery it is necessary that the battery or battery pack be periodically charged in order to ensure that they are maintained in a charged state. This generally alleviates the problem of having a dead battery at a time when its use is required. Another problem arises, however, in that a battery pack charger will generally require a relatively lengthy time to fully charge the batteries. It, therefore, has been known to maintain the battery in the charging mode during times of nonuse. However, this presents a further problem in that continued charging of an already charged battery pack leads to slow deterioration of the batteries.
To solve this problem, it is known in the art to have a fast charging system which will charge the battery pack in a relatively minimal amount of time so that the battery pack operator can charge the battery pack as it is needed. However, a number of problems arise even with this type of system. Since fast charging of a chargeable battery pack requires high current, it is extremely important to terminate the charging procedure before the battery pack is overcharged. In other words, the fast charging systems must have means for accurately indicating full battery charge. Charging beyond the full battery charge leads to battery deterioration and, over an extended number of overchargings, to battery pack failure.
A number of different systems are known in the art to detect full battery charge in a fast charging system. One method of determining full charge is by monitoring the temperature of the battery pack. These types of systems, however, suffer the drawbacks of repeated repetition of high temperature, low charging efficiency, and problems with safety in defective cells. A second type of fast charging system uses a voltage cutoff technique. These types of systems have proved to be unsatisfactory in that temperature variations lead to large voltage variations, and thus, an inaccurate full charge determination. Another type of fast charging system incorporates the termination of the charging as a function of the time of charging. These types of systems have been unreliable in that it is difficult to accurately tell what the state of the charge of the battery pack is at the initiation of the charging sequence.
A more reliable method of fast charging has been disclosed in which the charging device monitors the slope of the voltage-time curve for a particular battery. Since the voltage-time charging curve for a particular battery will always be substantially the same, it is possible to determine different points on the curve which represent different points in the charging sequence, and thus it is possible to determine which point of the curve represents full charge.
A quick charging system incorporating a type of slope monitoring technique is disclosed in U.S. Pat. Nos. 4,388,582 and 4,392,101 both to Saar et al. herein incorporated by reference and assigned to the same assignee as the present invention. The Saar et al. patents disclose a quick charging technique which analyzes the charging of a battery by noting inflection points which occur in the curve as the electrochemical potential within the battery changes with respect to time. By determining specific inflection points in the charging curve, it is possible to accurately terminate the rapid charging when the battery receives full charge.
The inflection point type analysis can be illustrated by viewing FIG. 1. FIG. 1 is a typical voltage-time curve of a nickel-cadmium ("NiCad") battery. As is apparent, the voltage continuously rises as the charging time increases until it gets to a maximum charge point. Although the specific values of the curve may differ from battery to battery, the general shape of the curve is typical for all nickel-cadmium batteries. Further, every type of rechargeable battery will have a voltage-time curve indicative of its type.
As is apparent, the curve can be separated into five distinct regions. Region I represents the beginning of the charging sequence. In this region, the voltage characteristics are somewhat unreliable and may vary from battery to battery in accordance with its prior history of being charged and discharged. It is for this reason that region I is shown as a dotted line. Further, this region is not important in the charging sequence since it is generally traversed within a relatively short period of time after the start of the charging sequence.
After approximately 30-60 seconds of starting the charging sequence, the charging curve will enter the more stable region of region II. Region II is generally the longest region of the charging sequence, and is marked by most of the internal chemical conversion within the battery itself. As is apparent, the voltage of the battery does not increase substantially over this region. At the end of region II is an inflection point A in the curve. Inflection point A represents a transition from region II to region III and is noted by a point where the slope of the curve changes from a decreasing rate to an increasing rate.
Region III is the region in which the battery voltage increases quite rapidly. As the battery reaches its fully charged condition, the internal pressure and temperature of the battery also increase substantially. When these effects begin to take over, the increase in battery voltage begins to taper off. This is noted as the inflection point B.
Region IV represents the fully charged region between inflection point B and the peak of the curve represented by point C. The voltage only stabilizes at point C for a short period of time. If charging continues, the additional heating within the battery will cause the voltage of the battery to decrease and, in addition, may damage the battery.
By analyzing the inflection points of the voltage-time curve, it can be determined at what point the battery has reached maximum charge. This is done by first determining inflection point A and then looking for inflection point B. Once inflection point B is observed, the charging process can be discontinued. Since it is possible to determine the inflection points very readily and accurately, it is possible to halt the charging process, or maintain the charging process at a maintenance charge, following detection of the second inflection point.
Although providing an effective and reliable method of preventing overcharging in a quick charger, the Saar et al, patents suffer the drawback that different battery packs having different rated voltage outputs cannot be charged with the same charging apparatus. What is needed then is an inflection point type quick charging device adaptable to different types of batteries having a relatively wide range of rated voltages. It is therefore an object of the present invention to incorporate the inflection point charging technique with a means for effectively and efficiently charging batteries at different rated voltage levels.